Dual mode heat pipes are used in a number of heating applications, such as heating of a concrete slab in a bridge or roadway. A liquid heat transfer medium or working liquid is evaporated in a primary evaporator portion of a sealed pipe or in both primary and auxiliary evaporator portions of the pipe. The energy-containing vapors move to a condenser or heat output portion of the pipe and are condensed therein. The sensible and latent heats of the vapors are transferred to the condenser portion and thus to the material (liquid, gas, or solid) surrounding the condenser portion by conduction and/or radiation. Heat energy from any source, such as solar, geothermal, electrical heating, flame, and the like, can be transferred essentially by conduction through the sealed walls of the evaporator portions of the heat pipe to heat and evaporate the working liquid. The primary evaporator of such a heat pipe is typically located in a "down pipe", that is, in a vertically oriented cylindrical portion of the heat pipe. The primary evaporator is located effectively below the condenser portion of the heat pipe and the condenser portion usually is located in a cylindrical portion of the heat pipe which is disposed at approximately a right angle to the "down pipe". The energy-containing vapor thus moves upwardly from the relatively hot primary evaporator into the relatively cool condenser portion where condensation of the vapor and heat transfer occurs. The condensate returns by gravity flow or by the capillary action of a wick to the "down pipe".
Auxiliary evaporators have previously been incorporated into such heat pipes to increase the total energy provided to the condenser portion of the heat pipes. Such heat pipes with an auxiliary evaporator are termed "dual mode" heat pipes. The auxiliary evaporators of prior heat pipes are typically located essentially in the condenser portion of the pipe and immediately below said condenser portion. Maintenance of the auxiliary evaporator is therefore a problem especially when the condenser portion of the heat pipe is located internally of a concrete slab, such as in the decking of a bridge. Further, the application of energy to the external surfaces of such auxiliary evaporators is often a problem and certain energy sources are not directly utilizable. Performance of auxiliary evaporators so disposed invariably degrades with time unless the primary evaporator continually functions. Since the vapors condense along the full length of the condenser portion, at least a portion of the working liquid is lost to the primary evaporator, with the effect of the auxiliary evaporator eventually "drying up" unless the primary evaporator functions for at least a certain determinable amount of time.
The deficiencies of prior dual mode heat pipes are obviated by the present structure. The heat is not only capable of indefinite operation through sole operation of the auxiliary evaporator, but is also capable of utilizing a wider variety of energy sources. The present heat pipe apparatus is also more easily maintained and is useful in situations where more than one energy source is available for utilization. The total amount of energy available to the condenser portion of a heat pipe is therefore increased.
The present dual mode heat pipe disposes the auxiliary evaporator coaxially of the primary evaporator at a portion thereof near the interface between the evaporator portion and the condenser portion of the heat pipe. As an example, in the heat pipes commonly used to heat the decking of bridges the evaporator portion is located within the "down tube" as aforesaid and the condenser portion is located in a cylindrical portion of the heat pipe relatively higher than and disposed at a right angle to the "down tube". The present auxiliary evaporator is located below the right-angle bend in the heat pipe and effectively above the primary evaporator. The auxiliary evaporator comprises an annular sleeve concentrically enclosing the upper end of the tubular primary evaporator. The annular sleeve of the auxiliary evaporator extends to form the condenser portion of the heat pipe. All of the condensed working liquid returns first by gravity or capillary action to the auxiliary evaporator. Liquid feed to the primary evaporator then occurs due to overflow of working liquid from the auxiliary evaporator into the upper end of the tubular primary evaporator. The present auxiliary evaporator is therefore an integral part of the "down tube" of the heat pipe. The auxiliary evaporator of the present invention is therefore not susceptible to "drying out" and can continue to function even in the absence of heat pipe function by the primary evaporator.
Accordingly, it is an object of the invention to provide a dual mode heat pipe wherein the auxiliary evaporator comprises an annular sleeve disposed concentrically and outwardly of the upper end of the "down tube" of the primary evaporator so that, condensed working liquid returns first to the auxiliary evaporator and then flows into the primary evaporator.
It is another object of the invention to provide a dual mode heat pipe wherein the auxiliary evaporator is positioned for easy maintenance thereof and more ready utilization of available energy sources.
These and other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.